Owing its excellent warmth-keeping effect, a carbonate spring has long been used in bathhouses and other facilities utilizing a hot spring. Basically, the warmth-keeping effect of a carbonate spring is believed to be based on the fact that the physical environment of human beings is improved owing to the peripheral vasodilative effect of carbon dioxide contained therein. Moreover, the percutaneous absorption of carbon dioxide causes an increase and dilation of the capillary bed and thereby improves blood circulation through the skin. Consequently, a carbonate spring is said to be effective for the treatment of degenerative diseases and peripheral circulatory disorders.
Since a carbonate spring has such excellent effectiveness, attempts have been made to prepare a carbonate spring artificially. For example, a carbonate spring has been prepared by bubbling carbon dioxide through a bath, by effecting the chemical reaction of a carbonate with an acid, or by sealing warm water and carbon dioxide in a tank under pressure for a certain period of time. Moreover, Japanese Patent Laid-Open No. 279158/'90 has proposed a method which comprises supplying carbon dioxide through a hollow fiber semipermeable membrane and thereby causing it to be absorbed into water.
Although a variety of apparatus for the preparation of a carbonate spring are now on the market, none of them are known to be capable of measuring and controlling the carbon dioxide concentration of the carbonate spring. One reason for this is that the carbon dioxide concentration of a carbonate spring is within a relatively low range, for example, of 100 to 140 ppm. However, since the effectiveness of a carbonate spring varies somewhat according to the carbon dioxide concentration, it might be desirable to prepare a carbonate spring having a higher concentration or a carbonate spring having a lower concentration.
A number of devices for measuring the concentration of carbon dioxide dissolved in water have conventionally been known. A carbon dioxide concentration meter of the flow type is composed of a carbon dioxide electrode and a carbon dioxide concentration indicator, but the diaphragm and internal fluid of the electrode must be replaced at intervals of 1 to 3 months. Thus, since this device requires troublesome maintenance and is rather expensive, it is not suitable for practical use as a measuring instrument in apparatus for the preparation of a carbonate spring. Carbon dioxide concentration meters of the thermal conductivity detection type, which are being used in apparatus for the preparation of carbonated drinks, are very expensive and unsuitable for the purpose of measuring the concentration of a carbonate spring.
A method for maintaining a constant carbon dioxide concentration in a bath by installing a pH sensor in the bath and controlling the feed rate of carbon dioxide supplied to the carbon dioxide dissolver is disclosed in Japanese Patent Laid-Open No. 215270/'96. However, owing to the influence of impurities dissolved in the carbonate spring within the bathtub or the quality of the raw water, a uniquely defined relationship between the pH and carbon dioxide concentration of the carbonate spring within the bathtub is not always established. Consequently, it is difficult to adjust the carbon dioxide concentration in a bath to a specified target value according to this method.